Imaging devices, search and track sensors and other types of optical devices which operate in the 3 -12 micrometer spectral region employ either reflective or refractive objective lens systems. An advantage of reflective optics is that wide spectral band operation is achievable. However, reflective optical systems have several undesirable features such as, for example, limited field of view and large physical size. In addition, central obscurations limits the diffraction modulation transfer function (MTF).
In the past refractive lens systems operating in the 3 -12 micrometer band have been designed for limited ranges, e.g., either the 3 -5 or the 8 -12 micrometer atmospheric windows. This is because the refractive indices of component lens materials vary significantly between the two atmospheric windows. For example, germanium, a common lens material for the 8 -12 micrometer wavelengths, operates like a "crown" in this portion of the spectrum while exhibiting characteristics of a "flint" in the 3 -5 micrometer region. Generally, the prior art has avoided dual range refractive optics because apochromatic systems designed for operation in one window have exhibited intolerable chromic aberrations in the other window.
The spectral sensitivities of infrared detectors and focal plane imaging devices are generally greater than the operating ranges of available refractive lens systems. This increased spectral sensitivity can be utilized at short ranges or high altitudes where atmospheric absorption is not a problem, but such applications have required use of reflective optics thereby imposing the above-mentioned limitations in wide band systems. It would be advantageous to provide a refractive lens system which is not limited to small bandwidths within the 3 -13 micrometer range.